Process for production of cuprammonium cellulose articles



Nov. 12, 1963 F. HOELKESKAMP 3,

PROCESS FOR PRODUCTION OF CUPRAMMONIUM CELLULOSE ARTICLES Filed Feb. 20, 1961 HOT AIR IOO- 20o INVENTOR' FRANZ HOELKESKAMP ATT'YS coagulating United States Patent C) 3,110,546 PROCESS FGR PRODUCTIGN F CUPRAMMO- NlUM CELLULGSE ARTELES Franz Hoelkeskamp, Wuppertal-Langerfeld, Germany,

assignor to .l. P. Bemberg Aktiengesellschait, Wuppcrtal-Gberharmen, Germany Filed Feb. 20, 1961, Ser. No. 96,167 Claims priority, application Germany Feb. 29, 1960 6 Claims. (Cl. 18-54) This invention is concerned with improvements in the production of cellulose articles such as filaments and films by the so-called cupramrnonium process, and in particular, the invention relates to a novel method of regenerating cellulose from the cuprarnmonium cellulose spinning solution. The production of filaments includes such structures as fibers and threads while the production of films is intended to include ribbons, tapes, foils and the like.

In the past, filamentary or filrnaceous cellulosic articles have been produced by a number of different procedures. The cuprammonium process was one of the first processes developed wherein a cellulose raw material is first decomposed and dissolved in a liquid solution which is then extruded or spun in the desired form. This extruded cellulose spinning solution is then coagulated and the cellulose regenerated by appropriate treatment.

The spinning solution in the cupramrnonium process is obtained by dissolving cellulose in a cuprammonium solvent which is essentially an aqueous solution of copper and ammonia characterized by the cupramrnonium ion. This aqueous solvent must contain specific quantities or concentrations of copper in cupric form and ammonia in order for the cellulose to remain dissolved therein. It is generally recognized that the cupramm'onium ion of the solvent forms a complex with the cellulose to give a soluble copper-ammonia-cellulose compound. This cuprarnmonium cellulose solution can then be extruded or spun, e.-g. as filaments or films into an aqueous spin bath or bath. The aqueous coagulating bath, which may be neutral or basic, essentially functions as a diluent to lower the concentration of ammonia and copper to the point where the cellulose tends to hardener coagulate in the form in which it is extruded. The spin bath may also serve to remove some of the solvent from the precipitated filaments or films.

Although it is possible to use water alone as the coagulating bath for the spun filaments or films, it has been a more common practice to employ an alkaline coagulating bath, in order to obtain a good coagulation and better qualities in the finished product. After coagulation, especially in an alkaline bath, there is believed to be formed a copperalkali cellulose material which can be decomposed to fully regenerate the cellulose by subsequent treatment in an acid bath. In some cases, particularly with films, it has been '"found advisable to treat the coagulated material from the allcaline bath with water alone in order to hydrolytically decompose the copper-alkali-cellulose prior to treatment with acid. The acid treatment then serves to remove copper hydroxide when taken in conjunction with subsequent washing to remove salts and adhering acid. After washing, the cellulose article is finally dried and stored inna suitable manner.

With respect to the production of filaments, the cuprammonium spinning solution is ordinarily extruded into a funnel or a bath through which the aqueous coagulating medium is circulated, and the cellulose of the coagulated filament is then regenerated by treatment in an acid bath,

washing and then drying. A portion of the ammonia and copper is removedduring coagulation, essentially by dilution of the spinning solution as noted above, and the monia and copper remaining with the cellulose after 2 coagulation are separated during the acid treatment and subsequent washing.

In the production of film, a preferred procedure according to the prior art is to extrude the cuprarnmoni-um spinning solution through a long narrow slot into an alkmine coagulation bath, e.g. ad-ilute sodium hydroxidesolution, in order to coagulate the film. Again, a portion of the ammonia and copper are removed during coagulation.

' The coagulated film is then treated with water for decomposition of the copper-alkali-cellulose and a further removal of ammonia. After this treatment with Water, the cellulose is saturated with copper hydroxide which can then be removed by treatment with acid followed by washing. i

In general, the spinning of cupnammonium filaments is accompanied by stretching or drawing in order to improve the tensile strength and other properties of the final product. Stretching is normally carried out together with the spinning of the solution intothe coagulation bath, but there are also a number of processes wherein the spun filaments are subsequently stretched to even a greater degree. Films may be also be stretched during their production, but it is quite common to extrude and treat the film without any substantial application of tension. It is further a common practice to add small quantities of stabilizing agents, softening agents or various finishing materials during the production of the cellulose articles, and these and other procedures will be well understood by those skilled in the art.

The cup'rammonium cellulose articles produced in accordance with the prior art are characterized by relatively large swelling values, i.e. a large absorption of water. Also, swelling of the cellulose occurs to a high degree in the aqueous baths employed in the actual production of filaments and films. As a result, the products of the cuprammonium process have a relatively loose or open structure, i.e. a relatively low density. However, such swelling during coagulation and regeneration of'the cellulose has been considered to be unavoidable since the various bath treatments are essential for regeneration of the cellulose and a substantially complete removal of ammonia and copper. For example, an incomplete reinoval of'ammonia often causes poor physical properties in the final product and may lead to misty or foggy spots which are particularly noticeable in films.

In commercial cuprammonium processes, it is quite desirable to recover as much copper and ammonia from the processes possible for economic reasons. The recovery of copper has been relatively easy, but very large amounts of ammonia are often discarded rather than attempting of misty'or foggy spots.

Yet another object of the invention is to provide an improved process of extruding and regenerating cellulose films and filaments from a cuprammonium cellulose solution wherein a novel method is employed for removal of ammonia from the extruded matei ial.

Still another object of the invention is to provide a process for producing cuprarnmonium cellulose films and filaments in which there is a higher recovery and reutilization of ammonia.

The manner in which these and other objects and advantages are obtained will be more fully understood in view of the following detailed description of the invention, taken in conjunction with the accompanying drawing wherein the sole figure is a schematic view, partly in crosssection, of suitable apparatus for carrying out the process of the invention.

It has now been found, in accordance with the invention, that it is possible to obtain a substantially improved cuprammonium cellulose process for the production of films and filaments if the cuprammonium spinning solution is extruded into an aqueous alkaline coagulating bath and led directly therefrom into a stream of a hot gas for a period of time sufficient to substantially remove ammonia from the coagulated product, but without substantial removal of water. The coagulated and gas-treated article is then subjected to treatment in an acid bath followed by washing and drying to remove copper and to complete the regeneration of the cellulose. The essential point of the invention thus resides in the removal of ammonia from the coagulated film or filament by subjecting the coagulated product for a short period of time to the action of a hot gas. This gas is preferably air for economical reasons but it will be understood that any hot gas can be employed does not chemically react with the cellulose. In general, the gas should be heated to a temperature of about 100 C. to 200 C., and is preferably maintained between about 110 C. and 175 C. Under these conditions, the normal period of hot gas treatment requires only a few seconds up to about one minute, depending upon the amount of ammonia to be removed and the thickness of the film or filament.

In general, a temperature of over 100 C. is necessary in order to substantially remove ammonia from the coagulated cellulose product within a reasonable time. Also, temperatures above 200 C. should generally be avoided so that the film or filament does not become damaged. Most importantly, substantial amounts or" water should not be evaporated from the coagulated product and in no case should the coagullated product be permitted to become completely dry. Within the above temperatures and treatment times, only a minimum of water is evaporated but ammonia is substantially eliminated from the coagulated product.

After treatment with the hot gas, such as air, the coagulated and substantially ammonia-free film or filament is in the form of a copper-alkali-cellulose which can then be treated with an acid bath followed by washing in order to dissolve and remove the copper content in the usual manner. Following this decopperizing with an acid bath and washing, the completely regenerated cellulose is dried by conventional means.

The process of the invention is further illustrated by a the drawing which discloses the manner in which a film or filament F is extruded or spun by conventional means 1 into a coagulating bath 2 and guided therethrough by the usual deflection rods or rollers 3 and 4. After the filament or film F leaves the coagulating bath 2, it is drawn through a pair of squeeze rollers 5 for removal of excess bath liquid and then through drying oven 6 by means of draw rolls 7. Additional draw rolls or guiding means 8 can be located within the oven.. Hot air at a temperature of 100-200 C. is introduced into the manifold 9 and enters the oven through nozzles or slots 10 so as to contact the surface of film or filament F. With a film, such nozzles or slots are preferably located on either side, i.e. above and below the film. The air and ammonia taken up from the film or filament are withdrawn through exit 11 and can be led to a condenser for recovery of ammonia. The fihn or filament, after treatment in the oven 6, is conducted through an acid bath in the usual manner as outlined above.

The cuprammonium cellulose spinning solution employed in the process of the invention can be prepared by well-known conventional methods from cotton linters,

1d wood pulp or similar raw materials. These known spin uing solutions generally contain copper in cupric form in a concentration of about 2 to 4% by weight and ammonia in a concentration of about 4 to 8% by weight, preferably 4.5 to 7%. The amount of cellulose in the spinning solu tion is usually about 8 to 12% by weight, water male'ing up the remaining portion of the solution. if desired, such solutions can be stabilized by the addition of Small amounts of glucose. I

This spinning solution is extruded from a conventional spinning or casting head having an aperture or a plurality of openings to form the desired film or filament. For ex ample, filaments can be spun through a number o fholes in a spinning head, these holes having a diameter of 1 mm. or less, preferably 0.10.3 mm. Films are produced by extrusion from a casting head with an aperture of any desired length and awidth corresponding to the thickness of the film which is ordinarily less than 7 mm., preferably about to -7 mm.

The desired shape or form of the product is extruded into an aqueous coagulating bath, which for the purpose of the present invention is preferably an alkaline bath, eg a 5 to 12% sodium hydroxide solution. The process of the present invention is not limited to the use of sodium hydroxide as the alkaline agent in the coagulating bath since the essential step of the invention involves the removal of ammonia by means of a hot gas in the regener ation of cellulose from the coagulated product. 7

As the coagulated film or filament passes through the coagulation bath, the spun cuprarnmonium cellulose solu-' tion is diluted and some of the ammonia remains with the bath. However, with the usual coagulation procedure, only about 76 to 82% of the total ammonia in the spinning solution is removed in the coagulating bath. in other words, as the film or filament is removed from the coagulating bath, it will ordinarily contain about 8 to 14% by weight of ammonia. If all of this remaining ammonia were not removed, there would be a noticeable influence on the final physical properties of the cellulose product.

The prior art has usually found it necessary to wash the coagulated product in water prior to the treatment in an acid bath for the purpose of removing further amounts of ammonia as well as to hydrolytically decompose the copper-alkali-cellulose. The removal of ammonia in this manner, however, is also accompanied by a very high degree of swelling of the hydrolytically released cellulose which is saturated with copper hydroxide. This swelling is disadvantageous because the resulting loose or low density structure of the cellulose remains even after the acid treatment. This structure cannot be noticeably improved during drying, and the resulting cellulose film or filament does not attain an optimum quality.

The ammonia removal step of the present invention is therefore advantageous because the ammonia content of the final product can be reduced to an absolute minimum while at the same time avoiding an extensive hydrolysis and swelling during the production of the cellulose object. This ammonia removal step essentially requires that thecoagulated film or filament be conducted through a stream of a hot inert gas such that substantially all of the ammonia is removed prior to treatment in an acid bath. Traces of ammonia can, of course, remain in the cellulose product, but it is preferable if the treatment with a hot gas reduces the ammonia content to not more than about 0.5% by weight based upon the final dry cellulose product. The water content of the film or filament as it leaves the coagulation bath. should not be reduced to any substantial extent during this treatment with a hot inert gas, and it is preferable to maintain the water content above a minimum value of about 45% by weight of the final dry cellulose product.

The ammonia removal in accordance with the invenan enclosed area such as a drying oven. The drying oven may contain inlet means and outlet means adapted to receive and eject the film or filament and spaced a distance apart so as to permit a proper time of treatment. The hot gas, such as hot air, can be directed from nozzles or elongated slots against the film or against a filament or plurality of filaments within the oven. The hot gas should pass :over the surface of the cellulose object, thereby removing ammonia, and the ammonia-containing gas can then be exhausted from the oven. Subsequently, the exhausted gas can be passed through a cooled condenser in order to condense and recover substantially all of the ammonia. This recovery of ammonia is an especially advantageous feature of the instant process as compared to prior washing treatments in which the ammonia is removed solely by washing baths. The recovered ammonia can then be reutilized for preparing fresh cuprammonium cellulose solution.

Following the removal of ammonia but only minor amounts of water with the hot inert gas, the film or filament in the process of the invention is led to the usual acid bath in which the copper is converted into a soluble compound. A final washing and drying then leads to a completely regenerated cellulose product. The conventional acid bath is ordinarily a dilute inorganic acid aqueous solution, for example an aqueous solution of sulfuric acid or hydrochloric acid. However, various organic acids or mixtures of acids can also be employed in this decopperizing bath.

The invention is further illustrated by the following examples, but it will be understood that the invention is in no way limited to the specific conditions of the examples. In each example, the prior art method of employing a water bath subsequent to coagulation is compared with the process of the present invention wherein a hot gas stream is substituted in place of the water bath. Percentages and parts are by weight unless otherwise specified.

Example 1 (a) A cuprammonium cellulose solution containing 9% by weight of cellulose, 6.5% by weight of ammonia and 3.9% by weight of copper was spun and coagulated as a filament having a denier of 150 in an aqueous 8% sodium hydroxide solution. The resulting copper-alkalicellulose filament was then conducted through a water bath in conventional manner for hydrolysis and then decoppered in an aqueous 8% sulfuric acid solution. The filamentary cellulose was next washed to remove salts and adhering acid and then dried.

(b) The same cuprammonium cellulose spinning solution as in the preceding paragraph was again spun and coagulated in an identical manner, but the water bath for treatment of the coagulated filament was omitted. Instead, the coagulated filament was exposed to an air stream of 100 C. for 45 seconds and then treated with the acid bath and washed and dried as in the previous experiment.

In order to maintain the simplest possible conditions, the filaments in both experiments (a) and (b) were spun without tension. The final cellulose filament in each case was then tested with respect to its properties of water absorption, tensile strength and elongation, the results being tabulated as follows:

(a) A cuprammonium cellulose solution containing 10% by weight of cellulose, 5% by weight of ammonia and 4.0% by weight of copper was extruded through a narrow slot with a width of 0.5 into an aqueous 8% sodium hydroxide solution in order to coagulate a copperalkali-cellulose This coagulated film was treated with water according to theprior art, then treated with acid and washed to decopperize the cellulose and finally dried. The acid treatment was carried out with an aqu ous 8% sulfuric acid solution.

(1)) The identical steps were followed as in the preceding paragraph except that the normal treatment with water was omitted, the coagulated film being exposed to hot air in place of this water treatment. The air temperature was 170 C., and three dilferent samples of film were exposed to the hot air for periods of 10, 20 and 45 seconds.

The film of experiment (a) and the three film samples of experiment (b) were measured for water absorption and burst strength with the following results:

Water absorption Burst (percent Strength by weight) (a) With water bath 139 7. 7 (b) With hot air:

(1) 10 seconds 123 8.4 (2) 20 seconds 120 8. 5 (3) 45 seconds 118 8. 7

In the tabulated results of the above experiments, the water absorption is measured as percent by weight of water based on a dry cellulose sample, i.e. grams of water per gram of dry cellulose multiplied by 100.

The burst strength is measured by first subjecting an area of 10 cm. film to a slowly rising air pressure until burst. This pressure in lag/cm. for the given film is converted to the corresponding pressure for a film with a thickness of 021 mm. a

It was quite surprising to find that in the process of the invention the use of a hot gas stream for removal of ammonia, even when the gas is heated to as high as 200 C., does not decompose or damage the cellulose article. Also, the method of ,the invention appears to maintain the copper-alkali-cellulose compound or the structure of the coagulated film or filament so that extensive swelling caused by hydrolysis does not take place. As a result, the surface of the cellulose article becomes much smoother and the resulting products has a higher density. These results are generally shown by improved values of swelling, i.e., lower swelling values. Furthermore, cellulose films manufactured according to the invention not only have a smoother surface but also show a greater impact resistance so that they cannot be easily damaged during the subsequent acid, water and drying treatments. The films also are less inclined to stick together and are therefore more easily handled with conventional apparatus. Both the improvement of the cellulose article and the higher recovery of ammonia in the process of the present invention represent substantial advantages from a commercial standpoint.

The invention is hereby claimed as follows:

1. In a process for the production of cellulose articles such as films and filaments wherein a cuprammonium cellulose solution is extruded into an aqueous alkaline coagulating bath and the coagulated product is subsequently treated with an acid bath, washed and remove copper and to complete the regeneration of the cellulose, the improvement which comprises: leading the extruded coagulated product directly from said alkaline coagulating bath into a stream of a hot inert gas maintained at a temperature of at least C. up to about 200 C. for a period of time sufiicient to substantially remove ammonia therefrom while retaining at least about 45% by weight of water therein, withreference to the final dry cellulose product, and thereafter treating the cellulose in said acid bath.

dried to 2. The process as claimed in claim 1 wherein air is employed as the hot inert gas.

3. The process as claimed in claim 1 wherein the hot gas treatment reduces the ammonia content of the cellulose to not more than about 0.5% by Weight, with reference to the final dry cellulose product.

4. The process as claimed in claim 1 wherein the cellulose article being treated is a filament with a diameter of about 0.1 to 1 millimeter.

5. The process as claimed in claim 1 wherein the cellulose article being treated is afilrn having a thickness of about 0.01 to 0.03 millimeter.

6. The process as claimed in claim 2 wherein air as the hot gas is maintained at a temperature of about 110 C. to 175 C.

References (Iited in the file of this patent UNITED STATES PATENTS 

1. IN A PROCESS FOR THE PRODUCTION OF CELLULOSE ARTICLES SUCH AS FILMS AND FILAMENTS WHEREIN A CUPRAMMONIUM CELLULOSE SOLUTION IS EXTRUDED INTO AN AQUEOUS ALKALINE COAGULATING BATH AND THE COAGULATED PRODUCT IS SUBSEQUENTLY TREATED WITH AN ACID BATH, WASHED AND DRIED TO REMOVE COPPER AND TO COMPLETE THE REGENERATION OF THE CELLULOSE, THE IMPROVEMENT WHICH COMPRISES: LEADING THE EXTRUDED COAGULATED PRODUCT DIRECTLY FROM SAID ALKALINE COAGULATING BATH INTO A STREAM OF A HOT INERT GAS MAINTAINED AT A TEMPERATURE OF AT LEAST 100*C. UP TO ABOUT 